2,497 research outputs found
Stellar Populations and the White Dwarf Mass Function: Connections To Supernova Ia Luminosities
We discuss the luminosity function of SNe Ia under the assumption that recent
evidence for dispersion in this standard candle is related to variations in the
white dwarf mass function (WDMF) in the host galaxies. We develop a simple
parameterization of the WDMF as a function of age of a stellar population and
apply this to galaxies of different morphological types. We show that this
simplified model is consistent with the observed WDMF of Bergeron et al. (1992)
for the solar neighborhood. Our simple models predict that WDMF variations can
produce a range of more than 1.8 mag in M(SN Ia), which is comparable to
the observed value using the data of Phillips (1993) and van den Bergh (1996).
We also predict a galaxy type dependence of M(SN Ia) under standard
assumptions of the star formation history in these galaxies and show that
M(SN Ia) can evolve with redshift. In principle both evolutionary and
galaxy type corrections should be applied to recover the intrinsic range of
M(SN Ia) from the observed values. Our current inadequate knowledge of the
star formation history of galaxies coupled with poor physical understanding of
the SN Ia mechanism makes the reliable estimation of these corrections both
difficult and controversial. The predictions of our models combined with the
observed galaxy and redshift correlations may have the power to discriminate
between the Chandrasekhar and the sub-Chandrasekhar progenitor scenarios for
SNe Ia.Comment: 20 pages, latex + 7 postscript figures, to be published in AJ,
September 199
Tunneling conduction in graphene/(poly)vinyl alcohol composites
Graphene/(Poly)vinyl alcohol (PVA) composite film with thickness
were synthesized by solidification of a PVA solution comprising of dispersed
graphene nanosheets. The close proximity of the graphene sheets enables the
fluctuation induced tunneling of electrons to occur from one sheet to another.
The dielectric data show that the present system can be simulated to a parallel
resistance-capacitor network. The high frequency exponent of the frequency
variation of the ac conductivity indicates that the charge carriers move in a
two-dimensional space. The sample preparation technique will be helpful for
synthesizing flexible conductors.Comment: 10 pages, 8 figure
Radiative corrections to the lattice gluon action for highly improved staggered quarks (HISQ) and the effect of such corrections on the static potential
We perform a perturbative calculation of the influence of dynamical HISQ
fermions on the perturbative improvement of the gluonic action in the same way
as we have previously done for asqtad fermions. We find the fermionic
contributions to the radiative corrections in the Luescher-Weisz gauge action
to be somewhat larger for HISQ fermions than for asqtad. Using one-loop
perturbation theory as a test, we estimate that omission of the fermion-induced
radiative corrections in dynamical asqtad simulations will give a measurable
effect. The one-loop result gives a systematic shift of about -0.6% in (r_1/a)
on the coarsest asqtad improved staggered ensembles. This is the correct sign
and magnitude to explain the scaling violations seen in Phi_B on dynamical
lattice ensembles.Comment: 10 pages, 5 figures. Minor corrections suggested by refere
The White Dwarf Population in NGC 1039 (M34) and the White Dwarf Initial-Final Mass Relation
We present the first detailed photometric and spectroscopic study of the
white dwarfs (WDs) in the field of the ~225 Myr old (log tau_cl = 8.35) open
cluster NGC 1039 (M34) as part of the ongoing Lick-Arizona White Dwarf Survey.
Using wide-field UBV imaging, we photometrically select 44 WD candidates in
this field. We spectroscopically identify 19 of these objects as WDs; 17 are
hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool
DC WD that exhibits no detectable absorption lines. We find an effective
temperature (T_eff) and surface gravity (log g) for each DA WD by fitting
Balmer-line profiles from model atmospheres to the observed spectra. WD
evolutionary models are then invoked to derive masses and cooling times for
each DA WD. Of the 17 DAs, five are at the approximate distance modulus of the
cluster. Another WD with a distance modulus 0.45 mag brighter than that of the
cluster could be a double-degenerate binary cluster member, but is more likely
to be a field WD. We place the five single cluster member WDs in the empirical
initial-final mass relation and find that three of them lie very close to the
previously derived linear relation; two have WD masses significantly below the
relation. These outliers may have experienced some sort of enhanced mass loss
or binary evolution; however, it is quite possible that these WDs are simply
interlopers from the field WD population. Eight of the 17 DA WDs show
significant CaII K absorption; comparison of the absorption strength with the
WD distances suggests that the absorption is interstellar, though this cannot
be confirmed with the current data.Comment: 24 pages, 13 figures. Accepted for publication in the Astronomical
Journal. Figures 1, 2 and 3 reduced in resolutio
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